Preventing overheating in classrooms: a thermodynamic simulation study in Barcelona
As climate change intensifies summer heatwaves, ensuring thermal comfort in educational buildings is becoming a top priority
Preventing overheating in classrooms: a thermodynamic simulation study in Barcelona


As climate change intensifies summer heatwaves, ensuring thermal comfort in educational buildings – particularly in Mediterranean cities like Barcelona – is becoming a top priority. In this post, we explore the results of a detailed thermodynamic simulation study assessing the risk of overheating in two classrooms of a primary school located in Barcelona.
The study was commissioned to understand how effectively a dedicated outdoor air system with an Air Halding Unit supplying a constant 14 °C, could maintain thermal comfort from May to September * with no additional zone thermostat controls or zone dampers. No optimisation of the thermal envelope was considered in the study.
* Schools shut in Spain from the end of June until September. However, for the simulations, July and August were included in the simulations, to have results under more challenging climate conditions. This was also done as the climate file is based on historical data. Currently, weather conditions that typically occurred in July now often occur in May.
Objectives of the Study
The simulation aimed to answer three key questions:
- How many hours during summer school hours do classrooms exceed 27 °C, despite cooled air being supplied at 14 °C?
- What is the optimal relationship between outdoor air temperature and supply air temperature to prevent both overheating and overcooling, with a lower limit of 14 °C for supply air?
- How does solar exposure differ between east- and west-facing classrooms, and how does this impact thermal comfort?
Simulation Tools and Model
- Software Used: Simulations were carried out using DesignBuilder with the EnergyPlus calculation engine.
- Climate Data: The IWEC II weather file for Barcelona–Airport, developed by ASHRAE, was used, based on long-term hourly data.
- Building Model: The model includes two identical classrooms—one facing east, the other west—located on the third floor of a school. The corridor between them runs north–south. Internal floors and walls were modelled as adiabatic.
- Envelope Performance: Building envelope parameters meet the minimum requirements of Spain’s CTE HE1 code for Climate Zone C (Barcelona). Windows are double glazed (Ug = 1.80 W/m²K) with a visible transmittance of 79% and a solar factor of 59%. The external wall has U = 0.49 W/m2·K, the roof has U = 0.40 W/m2·K and air permeability is n50 = 3 ach.
- Solar Shading: Each classroom includes external fixed shading devices with a 50% reduction factor, simulating expanded metal mesh (deployé).
Internal Conditions and HVAC Configuration
- Occupancy and Internal Loads: Each classroom is 60 m², with 31 pupils and one adult teacher. Lighting and equipment are only active during teaching hours (Mon–Fri, 08:00–18:00), though lighting is off in summer.
- Ventilation Strategy: A central AHU supplies 100% fresh air at a flow rate of 45 m³/h per person (1395 m3/h per classroom), with 79% sensible and 62% latent heat recovery efficiency.
- Cooling System: A air-water heat pump cools water to 7 °C, which is supplied to the AHU cooling coil, cooling the supply air to a constant 14 °C during summer school hours. The simulations assume this is also the respective zone supply air temperature (which in practice will not be the case due to heat gains/losses in the ductwork). In May and September, a dual-setpoint strategy is used to reduce overcooling:
· Outdoor T < 16 °C → Supply air at 20 °C
· Outdoor T > 17 °C → Supply air at 14 °C

Comfort Criteria
Thermal comfort was evaluated using the following thresholds:
- Too Cold: Operative temperature ≤ 22 °C
- Optimum comfort range: 22 °C – 27 °C, with minimum RH = 30% and maximum RH = 60% @ 26 ºC
- Too Hot: Operative temperature ≥ 27 °C
The analysis focused on school hours only, from May to September.
Key Results
External Climate Overview (08:00–18:00, Mon–Fri)
Minimum, average and maximum outdoor dry air temperatures for each of the months simulated are shown in the Figure below:

Overheating Risk – Summary (% of teaching hours > 27 °C)
The table below shows the overheating risk assessment for each classroom and month:

The East-facing classroom experienced overheating in July and September, principally due to morning solar gains. The West-facing classroom, by contrast, performed slightly better, with only 1% of overheating in September.
Overcooling Risk – Summary (% of teaching hours < 22 °C)
The table below shows the overcooling risk assessment for each classroom and month:

Slight overcooling was observed in May and September in the West classroom, highlighting the impact of delayed solar exposure in the morning. The following Figures show the results in graphic form for May, July and September.









Key Takeaways
- The central HVAC system generally maintains comfort during most of the school hours across both classrooms, despite the absence of zone-level temperature control.
- Orientation matters: The East classroom heats up faster in the morning, while the West classroom is more prone to overcooling early in the day, especially in shoulder seasons.
- July presents the greatest overheating risk, with internal operative temperatures exceeding 27 °C in the East classroom during 7% of teaching hours, despite the 14 °C supply air.
- Overcooling is avoided through a well-designed dual-setpoint strategy in May and September, where supply air is adjusted depending on outdoor temperature.
- Accurate commissioning is essential. Given the centralised nature of the system, delivering the correct airflow and temperature to each classroom is critical to ensuring real-world performance matches the simulation.
Final Thoughts
This simulation underscores the importance thoughtful but simple HVAC control logic in preventing thermal discomfort in classrooms. In warm climates like Barcelona, small improvements in system design and control can make a significant difference in educational outcomes and energy efficiency, using simple controls. For space cooling with 100 % outdoor air systems in larger buildings with long ventilation duct runs, it’s important to consider pressure losses and heat gains as cool air moves through the ductwork, and how this may impact flow rates and air temperature at the supply air grilles in each classroom.
Although not included in the study, it’s also essential to include and optimise passive design strategies (thermal insulation, window specification, airtightness, external shading devices, external “cool colours” and reduction of internal heat gains) to improve thermal comfort and reduce energy consumption.
As we move toward more resilient and climate-adaptive buildings, this study provides a clear example of how digital tools like EnergyPlus and DesignBuilder can inform evidence-based design decisions.